Action: Can nest protection increase predation of adults and chicks?

Key messages

Three replicated and controlled studies from North America and Sweden found higher levels of predation on adult birds with nest exclosures, one study from Sweden found that predation was no higher.

A replicated and controlled study from Alaska found that long-tailed jaegers Stercorarius longicaudus learned to associate exclosures with birds, targeting adult western sandpipers Calidris mauri and quickly predating chicks when exclosures were removed.

Background information and definitions

Nest cages and individual barriers are distinctive and may attract predators if they learn to associate the cages with potential prey. As with the previous section, unless studies specifically mention increased predation, they are discussed in ‘Physically protect nests with individual exclosures/barriers’.

Supporting evidence from individual studies

1

A replicated, controlled study from 1993-2002 at five alkali lakes in Alberta and Saskatchewan (Canada), Montana and North Dakota, USA (Murphy et al. 2003) found that adult piping plovers Charadrius melodus were more likely to be predated if their nests were protected by exclosures (5% of 1,355 nests suffering mortality) than if their nests were unprotected (no adults predated at 420 nests). Predation rates were highest (up to 48%) at sites with 4–15% tree cover within 2 km of the nests and zero in areas with few trees (across the study period, 393 nests monitored). At one site, when small (1-1.7 m diameter) exclosures were replaced with large (3-4 m diameter) ones with netting tops, predation rate fell from 34% in 1999 (55 nests) to 11% in 2000 (39 nests) In areas where large cages only were used, predation rates were 0.7% (303 nests). Most (78%) losses were to raptors.

2

A replicated before-and-after study onbeaches in California, USA (Neuman et al. 2004) found that nest abandonment rates of snowy plovers Charadrius alexandrinus combined with adult mortality increased between 1984-90 and 1991-99 (1% of 728 nests in 1984-90 vs. 4% of 682 in 1991-99) following the protection of 49% of nests with predator exclosures (1.5 m high triangular wire fences) after 1991. In addition, although only 49% of nests were protected, 75% of adult disappearances (assumed to be due to predation) were from protected nests (significantly more than expected by chance). This study is also discussed in ‘Predator control not on islands’, ‘Physically protect nests with individual exclosures/barriers’ and ‘Can nest protection increase nest abandonment?’.

3

A replicated and controlled study in 2001 in the Yukon Delta, Alaska, USA, (Niehaus et al. 2004) found that survival of western sandpiper Calidris mauri nests was higher when they were protected by exclosures (see ‘Physically protect nests with individual exclosures/barriers’). However, after 17 days, long-tailed jaegers (skuas, which predate on sandpiper adults, chicks and eggs) Stercorarius longicaudus began associating exclosures with nests and targeting them (whilst ignoring control nests), causing sandpipers to flush, sometimes colliding with the exclosures. One chick died from cold exposure whilst adults were being harassed by jaegers and exclosures were removed after 19 days. Following exclosure removal, chicks from exclosure nests were less likely to survive than those from control nests, with some chicks being predated minutes after the removal of exclosures.

4

A replicated, randomised and controlled trial in 2002 and 2004 at three grazed pasture sites in south-west Sweden (Isaksson et al. 2007) found that there were significantly higher predation rates on adult common redshank Tringa tetanus with protected nests (protected by truncated cone steel cages with 6.5 – 8.5 cm spacing between vertical bars and 4 x 4 cm steel netting on top) than for birds brooding at unprotected nests (nine adults from eight protected nests predated, from a total of 37 nests vs. a single bird from 31 unprotected nests). This study is also discussed in ‘Physically protect nests with individual exclosures/barriers’.

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Effectiveness

An assessment by independent experts of the effectiveness of this action based on the summarized evidence (0% = not effective, 100% = highly effective). This score is based on the direction and size of the effects reported in each study. Actions with high scores typically have large, desirable effects on the target species/habitat in each study. There is some variation between actions, e.g. 100% effectiveness in adding underpasses under roads for bat conservation will likely have different impacts to 100% effectiveness in restoring marsh habitat. The effectiveness score does not consider the quantity or quality of studies; a single, poorly designed study could generate a high effectiveness score. The effectiveness score is combined with the certainty and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Harms

An assessment by independent experts of the harms of this action to the target group of species/habitat, based on the summarized evidence (0% = none, 100% = major undesirable effects). Undesirable effects on other groups of species/habitats are not considered in this score. The harms score is combined with the effectiveness and certainty scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Certainty

An assessment by independent experts of the certainty of the evidence for this action based on the summarized evidence (0% = no evidence, 100% = high quality evidence). How certain can we be that the effectiveness score applies to all targets of the intervention (e.g. all birds for an action in the bird synopsis)? This score is based on the number, quality and coverage (species, habitats, geographical locations) of studies. Actions with high scores are supported by lots of well-designed studies with a broad coverage relative to the scope of the intervention. However, the definition of "lots" and "well-designed" will vary between interventions and synopses depending on the breadth of the subject. The certainty score is combined with the effectiveness and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Overall Effectiveness Category

The overall effectiveness category is determined using effectiveness, certainty and harms scores generated by a structured assessment process with multiple rounds of anonymous scoring and commenting (a modified Delphi method). In this assessment, independent subject experts (listed for each synopsis) interpret the summarized evidence using standardised instructions. For more details see https://www.conservationevidence.com/content/page/79.